1. Field of the invention
The invention relates to frequency standards based on mode-locked fiber lasers, fiber amplifiers and fiber-based ultra-broad bandwidth light sources, and applications of the same.
2. Description of the Prior Art
Mode-locked fiber lasers have several advantages over both mode-locked bulk solid state lasers and mode-locked diode lasers. Mode-locked fiber lasers offer typically superior noise properties compared to mode-locked diode lasers and can be packaged in smaller spaces than mode-locked bulk solid state lasers. Mode-locked fiber lasers can be produced with excellent thermal and mechanical stability. Passively mode-locked fiber lasers in particular can be constructed with few and inexpensive optical components, suitable for mass production, as disclosed in patent publication US 2004/0213302 to Fermann et al.
The dispersion compensated fiber lasers as disclosed by Fermann et al. allow the construction of low noise frequency comb sources. The incorporation of highly nonlinear Bi-fibers was further suggested to minimize the noise of such sources. In addition Fermann et al. disclosed the design of fiber lasers operating at repetition rates in excess of 1 GHz.
Low-noise operation of fiber lasers minimizes their timing jitter, allowing optimized control of the timing of the pulses. In contrast, other prior art modelocked fiber lasers are either limited in repetition rate or achievable timing jitter.
Generally, the noise background in fiber lasers can be minimized by operating well above threshold. Hence, modelocked fiber lasers enabling the oscillation of high energy pulses are very beneficial. Such high pulse energy fiber lasers were previously described in U.S. application Ser. No. 11/109,711 to Fermann et al. based on the oscillation of near parabolic pulses inside a positive dispersion fiber laser cavity. An optimum in pulse stability is then obtained by the incorporation of bandpass filters into the cavity. As mentioned in U.S. application Ser. No. 11/109,711 the bandpass filtering action can also come from the gain medium. The benefits of using parabolic pulses inside modelocked fiber lasers were later reiterated in US patent application 2005/0169324 to Ilday et al. The lack of any bandpass filters in these systems restricts the operation of such systems to parabolic fiber lasers operating close to the zero dispersion point and can lead to pulse stability problems.
The '302 publication constituted the first low noise fiber-based frequency comb source. Here low noise operation was obtained by controlling the fiber cavity dispersion in a certain well-defined range. Low noise operation of fiber frequency comb sources is generally required in order to enable stable locking of the carrier envelope offset frequency fceo of the laser. For example, early work on fiber frequency comb lasers as disclosed in F. Tauser, A. Leitenstorfer, and W. Zinth, “Amplified femtosecond pulses from an Er:fiber system: Nonlinear pulse shortening and self-referencing detection of the carrier-envelope phase evolution,” Opt. Express 11, 594-600 (2003) did not allow locking of fceo of the laser. Other related work by Tauser et al. in US patent publication 2004/0190119 also does not teach a method of producing a low noise frequency comb source.